1. Field of the Invention
The present invention relates to an optical switch device, and more particularly to a multiport optical switch device for switching one or at least two input light beams to one or at least two of a multiplicity of resolvable output light beams at different output locations.
2. Description of the Prior Art
Optical switches are known which utilize an acousto-optic effect. Such switches utilize the Bragg diffraction, i.e. the phenomenon that an incident light propagating through an optical waveguide layer on a piezoelectric substrate, for example, of LiNbO.sub.3 is diffracted by a surface acoustic wave (hereinafter referred to as "SAW") propagating on the waveguide layer. The frequency of the SAW, when varied within a suitable range, e.g. in -3 dB bandwidth, alters the deflection angle of the light beam, so that the desired one of resolvable deflected light beams can be selected. To obtain a multiplicity of resolvable deflected light beams, it is required that the frequency of the SAW be varied over a wide range and that each of the individual lights within the aperture of the incident light beam satisfy the Bragg condition with the SAW of one of the frequencies.
Applied Physics Letters, Vol. 26, No. 4, pp. 140-142, Feb. 15, 1975 discloses such a multiport switch under the title "High-performance acousto-optic guided-light-beam device using two tilting surface acoustic waves." The acousto-optic guided-light-beam device includes two SAW interdigital transducers (hereinafter referred to as "IDT's") which are arranged on a waveguide layer and tilted to generate two SAW's propagating in the tilted directions, the transducers being designed to have different center frequencies. The tilting angle between the two SAW IDT's is determined by the difference in the Bragg angles at their center frequency. Reportedly the two transducers have the designed center frequencies of 255 and 382 MHz, respectively, and a tilting angle of approximately 0.3.degree.. The two IDT's are connected in parallel. To be sure, the device provides a multiplicity of resolvable output light beams because the deflection angle of a light beam is variable over a wide range. However, the two IDT's, which must be tilted accurately at a very small angle as described above, are very difficult to fabricate on the optical waveguide layer. The device further involves the problem that since the frequency of the signal to be applied to the two IDT's is varied over a wide bandwidth, it is difficult to match the high-frequency wave generator with the IDT's properly.